Exercise generally obeys the normal rules of mathematics. You can replace one 40-minute workout with two 20-minute bouts, or even four 10-minute bouts, and get roughly the same health benefits. But beyond that, the rules break down: Exercise in bouts lasting less than 10 minutes simply doesn't count.

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At least, that's what exercise physiologists and public-health authorities have been telling us for years.

But influential groups such as the American College of Sports Medicine are now reconsidering the value of ultra-short bouts of activity, and a new Canadian study suggests that the gradual accumulation of "incidental physical activity" - sweeping the floor, taking the stairs - in bouts as short as one minute can also contribute to your cardiovascular fitness level.

Recognizing the role of these micro-bursts of activity should encourage people who are currently sedentary and find the prospect of structured exercise daunting, says Ashlee McGuire, a recent PhD graduate in kinesiology and health studies at Queen's University in Kingston.

"I do think that the idea of a '10-minute minimum' is a barrier for some people," she says. "I'd like to see more focus on doing little bits of physical activity throughout the day."

To study the issue, Dr. McGuire and her supervisor, Robert Ross, outfitted 135 obese, inactive volunteers with small, hip-mounted accelerometers around the clock for a week. They then analyzed the activity patterns, adding up the total amount of sporadic "light" and "moderate" movements in one-minute sections, and compared the data to the results of a treadmill test of cardiovascular fitness. The results appear in the journal Medicine & Science in Sports & Exercise.

The least-active third of the group accumulated just six minutes of moderate activity throughout each day. The middle-third racked up 17 minutes, and even the most-active third accumulated a still-modest total of 34 minutes per day of moderate activity. Despite the fact that none of the subjects did any "exercise" that met that 10-minute minimum threshold, the most-active group was significantly fitter than the least active group, by an amount corresponding to a 13-per-cent reduction in risk of death from all causes and a 15-per-cent reduction in risk of heart disease.

These findings contradict the American College of Sports Medicine's 1998 official position stand on physical fitness, which asserted unequivocally that physical activity lasting less than 10 minutes is "not a sufficient stimulus for developing and maintaining fitness in healthy adults."

Since then, though, researchers have begun to reconsider the idea of a minimum threshold for exercise. In part, the debate reflects the challenges of crafting one-size-fits-all public health messages for a diverse population: The fitter you are to start with, the greater the amount of exercise you need to get any benefit from it.

Dr. McGuire is careful to point out that even the most active subjects in her study were still obese and at an elevated risk of a wide variety of health problems. For the 60 per cent of Canadian adults who are in a similar situation - inactive, and for whatever reason unwilling or unable to embark on a daunting exercise plan - accumulating two minutes of physical activity here and three minutes there can make a real difference to their health. But it's still nowhere near as beneficial as hitting the standard recommendation of 30 minutes of moderate exercise five times a week.

Last month, a panel of eight leading researchers released an updated ACSM position stand to replace the 1998 document. Their discussion of the minimum threshold reflects a more nuanced understanding of the needs of different people:

"Durations of exercise [less than]10 minutes may result in fitness and health benefits, particularly in sedentary individuals," they wrote. "However, the data are sparse and inconclusive."

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